Do you need interrupt control, or can you program your serial devices to
communicate using an address scheme and one serial port i.e.
transmitting a key word that the device will act on.

If not then use a serial card to communicate using many com ports mapped
in a table. Likely Linux will map any serial ports you have installed
on your box. I don't know this for sure, but I can't believe it fails
to register all serial ports.

Each different device has it's own control language, as does a modem. I
like using the PIC line of controllers myself, with my own programmed
control words to wake them up. They listen on a single serial bus and
act on commands when they hear their key word for the next command chain.

Like I said, if your devices are not standard, communication should be
supplied by the manufacture in the form of a driver with documentation
for communicating with them. If they are standard in the form of
"Popular" the drivers may be in the kernel. So, what you trying to do?